Usage of gRPC channel options is not compatible with Python 3.8

[mschmengler-vmray]

When using the gRPC Client, the optional parameter channel_options is available to pass gRPC-channel related options to the client.

As of version 0.10.4 of the Cerbos SDK, the processing of these options uses the Python-3.9 operator |= for dicts.
This makes it currently impossible to use the gRPC channel_options with a Python-3.8 codebase and kinda also breaks the intended compatibility with Python 3.8 in general.

In cerbos/sdk/_sync/_grpc.py, line 126, the statement options |= channel_options should be replaced with options.update(channel_options) or something equivalent.

Also consider to update the docs not to use "grpc.service_config" in the channel_options, because dict.update() overrides existing keys, and "grpc.service_config" is already being used by the Cerbos SDK.